Racket or club consisting of magnesium

ABSTRACT

The present invention relates to a frame for a ball-game racket comprising a head portion and a handle portion, wherein the frame is configured as a hollow profile and comprises magnesium and wherein the wall thickness of the hollow profile varies along a cross-section through the hollow frame profile, as well as to a process for producing such a racket.

The present invention relates to a frame for a ball-game racket comprising magnesium as well as to a process for producing such a frame.

A variety of different materials have already been proposed as material for the frame of a ball-game racket, in particular a tennis racket. After initial wood rackets, however, the only rackets that have become technically accepted are aluminum rackets whose frame is bent from an aluminum profile and glass-fiber reinforced rackets and in particular carbon-fiber reinforced rackets whose frame is formed from one or more so-called prepreg tubes.

Furthermore, it is known from, for example, U.S. Pat. No. 3,874,667 to use magnesium as material for the frame of ball-game rackets. The racket disclosed in U.S. Pat. No. 3,874,667 consists of a solid metal frame. Such a solid metal frame cannot meet the current standard requirements concerning weight and playability of a ball-game racket.

If, however, the frame for a ball-game racket is produced by the tube blowing process from prepreg layers, as nowadays is usually the case, it is highly sophisticated to vary the wall thickness of the frame in a targeted manner in order to exert influence on the local rigidity of the frame and the weight distribution of it. On the other hand, if the frame is bent from a hollow aluminum profile, the wall thickness of the frame profile is typically constant since aluminum profiles having a constant cross-section are used.

The present invention is based on the problem of providing a frame for a ball-game racket and a process for producing such a frame which make it possible to optimize the local rigidity of the frame in a targeted manner via the variation in the wall thickness and to achieve an optimum weight distribution, wherein the total weight of the frame is to be kept as low as possible.

This problem is solved by a frame for a ball-game racket according to claim 1 and a process for producing a frame for a ball-game racket according to claim 17. The frame for a ball-game racket according to the invention, in particular for a tennis racket, comprises a head portion and a handle portion. The frame is configured as a hollow profile and comprises magnesium, wherein the wall thickness of the hollow profile varies along a cross-section through the hollow frame profile.

In the context of the present invention, the term “hollow profile” means a really hollow profile which is exclusively formed by a frame wall encompassing a hollow space, wherein said hollow space is free from additional walls, struts and the like. In other words, mechanical properties such as flexural rigidity and natural frequencies are determined according to the invention essentially exclusively by the variable wall thickness of the hollow profile without, for example, any additional stabilizing struts being necessary.

Within the scope of the invention, it turned out that magnesium and/or magnesium alloys can be used for a hollow frame profile for a ball-game racket. The production techniques used in this connection advantageously permit a variation in the wall thickness of the hollow profile in a targeted manner, in particular also along a cross-section through the hollow frame profile. In the context of the present invention, the cross-section through the hollow frame profile means a cut perpendicular to the contour of the frame profile. A targeted variation in the wall thickness along such a cross-section through the hollow frame profile is almost impossible with the commonly used tube blowing process since the prepreg layers are generally wound such that they extend along such a cross-section through the hollow frame profile. With the tube blowing process, it is typically only possible to realize different cross-sectional shapes and/or dimensions.

The frame according to the invention can be produced, for example, by means of magnesium injection-molding techniques such as, for example, the so-called thixo process. When using this technique, the wall thickness can be simply varied in a targeted manner by respectively configuring the injection mold. In this connection, the so-called lost core technique, in which an injection mold core is molten after the frame has been removed from the mold, is particularly preferred.

It is further preferred that the wall thickness of the hollow profile does not only vary along a cross-section through the hollow frame profile but additionally also varies in the longitudinal direction of the frame, i.e. along the frame contour. The wall thickness in the throat portion, i.e. in the transition between the bridge and the head portion of the frame, is larger than in the remaining head portion.

According to a preferred embodiment, the variation in the wall thickness along at least one cross-section and/or longitudinal section relative to the minimum wall thickness is at least 25%, preferably at least 50%, more preferably at least 75% and particularly preferably at least 100%.

When using magnesium and/or magnesium alloys, particularly thin wall thicknesses can be achieved in particular by means of magnesium injection molding. According to a preferred embodiment, for example, the wall thickness of the frame according to the invention in sections is thinner than 1 mm, preferably thinner than 0.8 mm, more preferably thinner than 0.7 mm and particularly preferably thinner than 0.6 mm.

In the head portion, the frame profile of the frame according to the invention comprises preferably an outer portion facing outwardly, an inner portion facing inwardly towards the stringbed, and two side portions, wherein the wall thickness of the inner and/or outer portion at least in sections is thicker than the wall thickness of the side portions. In this connection, the frame profile does not have to be rectangular. Rather, further portions may form a smooth transition from the inner and/or outer portion to the two side portions so that a polygon, a rectangle having rounded edges or the like is formed. According to an alternative preferred embodiment, the wall thickness of the inner or outer portion at least in sections is thinner than the wall thickness of the side portions. Additionally or alternatively, the wall thickness of the inner portion at least in sections may be thinner or thicker than the wall thickness of the outer portion.

The frame preferably comprises at least 50% by weight, more preferably at least 80% by weight, more preferably at least 90% by weight and particularly preferably at least 95% by weight of magnesium. According to a preferred embodiment, the frame comprises a magnesium alloy, preferably a fiber-reinforced or particle-reinforced alloy, particularly preferably SAE SiC/AZ91. It is particularly preferred that essentially the entire frame is made of such an alloy. Additionally, the frame can be locally reinforced with carbon fiber material and/or glass fiber material. Such a reinforcement is preferably provided in the area of the transition between the head portion and the handle portion.

According to the invention, the magnesium proportion and/or magnesium-alloy proportion does not have to be distributed uniformly across the ball-game racket frame. Rather, it is also preferred that the frame has a frame portion comprising magnesium, whereas the wall thickness of the hollow profile along a cross-section through this frame portion varies. In this case, the frame in this frame portion comprises preferably at least 50% by weight, more preferably at least 80% by weight, even more preferably at least 90% by weight and particularly preferably at least 95% by weight of magnesium. In this preferred embodiment, one or more components can be separately made from, in particular injection molded from, magnesium or a magnesium alloy. These components can then be inserted together with the prepreg layers in a tube blowing mold and pressed together with the prepreg layers. These separate components can be individual portions of the head portion or the handle portion. According to the invention, however, it is particularly preferred that a separate throat portion is provided which consists of magnesium or comprises magnesium or magnesium alloys, since in this area particularly advantageous effects can be achieved by means of a respective variation in the wall thickness.

The frame portion comprising the magnesium preferably consists of two frame portion halves which are welded to each other, pressed together and/or bonded to each other. When the entire frame comprises magnesium, preferably the entire frame consists of two frame halves which are welded to each other, pressed together and/or bonded to each other. The frame halves or the frame portion halves are preferably identical, wherein each of the two frame halves or frame portion halves is asymmetrical with respect to the longitudinal axis of the frame or the frame portion. In this way, it is possible to produce in a particularly simple manner two identical halves by means of injection molding, said two halves engaging with each other when being connected.

The present invention further relates to a process for producing a frame or frame portion for a ball-game racket. The frame produced according to the process of the invention is preferably a frame exhibiting the above described properties. According to a preferred embodiment of the process for producing a frame or frame portion according to the invention, an injection mold and a core are provided, wherein the core preferably consists of a material having a melting temperature below the melting temperature or heat resistance temperature of the frame material. The melting temperature of the core material is preferably lower than 400° C., particularly preferably lower than 370° C. It is further preferred that the melting temperature of the core material is above 190° C. and particularly preferred that it is above 200° C. The frame (or a frame portion) is then formed, preferably by means of the thixo process, by inserting a material comprising magnesium between the injection mold and the core. The core material is selected such that during unmolding the frame thermal energy not sufficient to fuse the core is introduced into the core, wherein slight softening can possibly be tolerated. Once the frame has been formed, removed from the mold and cooled to such an extent that it stably maintains its shape, the core is molten, preferably by heating the entire racket to above the melting temperature of the core, so that the core material can escape from the hollow space formed by the frame. Preferably, the heat resistance temperature of the frame is not exceeded in this operation.

The core material preferably comprises or consists of one of or a combination of the following materials: alloys on the basis of tin (preferably at least 50% of tin and optionally zinc, bismuth, antimony, copper, silver, lead, indium), alloys on the basis of bismuth (preferably at least 50% of bismuth and optionally tin, silver, germanium), alloys on the basis of lead (preferably at least 50% of lead and optionally tin, antimony, bismuth, silver, indium), alloys on the basis of zinc (preferably at least 50% of zinc), die-cast zinc alloys such as, for example, ZnAl4, ZnAl4CuI, ZnAl4Cu3, alloys on the basis of indium, glass, plastics with or without glass-fiber reinforcement such as, for example, polyamide. Metallic integral foam cores on the basis of these low-melting alloys are particularly preferred. They can be produced, for example, by means of die casting. An integral foam comprising a closed skin is formed by the addition of a foaming agent. The relative density can be, for example, 50% to 100% of the absolute material density. Thus, by using a foam, only half of the material amount is required in comparison to a solid core.

According to a further preferred embodiment of the process for producing a frame according to the invention, an injection mold and a core are provided, wherein the core consists of a salt. The frame (or a frame portion) is then formed, preferably by means of the thixo process, by inserting a material comprising magnesium between the injection mold and the core. Once the frame has been formed, removed from the mold and cooled to such an extent that it stably maintains its shape, the core is washed out from the hollow space formed by the frame and/or dissolved by introducing water into the hollow space formed by the frame.

According to a further preferred embodiment of the process for producing a frame according to the invention, the core can also consist of an acid-soluble material, e.g., on the basis of glass, so that upon molding the frame the core can be removed from the frame by means of an acid.

According to a further preferred embodiment of the process for producing a frame according to the invention, firstly two frame halves are cast from a material comprising magnesium. Preferably, this is made by means of the so-called thixo process. Subsequently, the two frame halves are connected to form a frame.

In the process of the invention for producing a frame portion according to this second embodiment, firstly two frame portion halves are cast from a material comprising magnesium and then the two frame portion halves are connected to form a first frame portion. In order to produce a frame for a ball-game racket from this first frame portion, a second frame portion is provided and the first and second frame portions are connected to form a frame. According to the invention, the second frame portion may be an unfinished portion, for example a prepreg tube, so that the completion of the second frame portion and the connection of the second frame portion to the first frame portion take place in one process step.

Injection molding the material comprising magnesium is preferably carried out at temperatures between 500° C. and 800° C., particularly preferably at temperatures between 550° C. and 700° C. The injection of the material into the injection mold is preferably performed in less than 80 ms, particularly preferably in less than 60 ms and particularly preferably in less than 50 ms.

The step of connecting the two frame halves or frame portion halves preferably comprises welding the halves to each other and/or pressing them together and/or bonding them to each other. Furthermore, the two frame halves or frame portion halves are preferably identical, wherein each of the two frame halves or frame portion halves is asymmetrical with respect to the longitudinal axis.

The frame for a ball-game racket according to the invention comprises a number of technical advantages over the prior art. Although magnesium has a higher density, a lower specific rigidity and a lower specific strength than usual prepreg materials, a frame for a ball-game racket of magnesium and/or magnesium alloys can be better optimized as regards rigidity and strength while having the same weight, since the wall thickness can be optimally configured in a simple way. The use of magnesium injection molding permits an automated production comprising low cycle times of about 35 seconds. A pretreatment and an aftertreatment, which are often manually performed in the conventional prepreg processes, are almost completely superfluous. Injection molding further permits an excellent production stability and permits to produce exactly the desired structure by means of a defined injection mold.

In the following, preferred embodiments of the frame according to the invention are described in more detail by means of the Figures, in which:

FIG. 1 shows a perspective view of the frame according to the invention;

FIG. 2 shows a perspective view of a frame half of the frame according to FIG. 1;

FIG. 3 shows an enlarged perspective view of the throat portion of the frame according to FIG. 2;

FIG. 4 shows a perspective view of the transition from the handle portion into the throat portion of the frame according to FIG. 2;

FIG. 5 shows a perspective view of a portion of the head portion of the frame according to FIG. 2;

FIG. 6 shows a perspective cross-section through the head portion of the frame according to FIG. 1;

FIG. 7 shows a perspective cross-section through the bridge of the frame according to FIG. 1;

FIG. 8 shows a perspective longitudinal section through the handle portion of the frame according to FIG. 1;

FIG. 9 shows a top view on a separate component for a frame according to a preferred embodiment of the present invention;

FIG. 10 shows a side view as viewed from the stringbed plane onto the component of FIG. 9;

FIG. 11 shows a side view from a side opposite FIG. 10 onto the component according to FIG. 9;

FIG. 12 shows a perspective view of the component according to FIG. 9; and

FIG. 13 shows a further perspective view of the component according to FIG. 9.

FIG. 1 shows a perspective view of a frame for a ball-game racket according to a preferred embodiment of the present invention. The frame is configured as a hollow profile and comprises a head portion 1 for accommodating strings that are not shown, a handle portion 2 and a throat portion 3 comprising two branches 4 a and 4 b as well as a bridge 5. The frame of this preferred embodiment is composed of two identical frame halves which are asymmetrical with respect to the longitudinal axis of the frame. A perspective view of one of these two identical frame halves is shown in FIG. 2.

As apparent from FIG. 2, the asymmetry of the frame halves is such that the two frame halves can engage with each other when being joined. This can be seen particularly clearly in the enlarged detail according to FIG. 3 illustrating the throat portion 3 of the frame half of FIG. 2: The projecting ridges 5 a of the bridge 5, for example, can engage with the stepped portions 5 b of the bridge 5 of the other frame half. This enables a flush connection of the two frame halves to form a frame. The head portion (cf. FIG. 5) similarly comprises areas with projecting ridges 1 a and stepped portions 1 b.

In the illustrated preferred embodiment, the step forms a kind of limit stop for the second frame half, and the ridge and step can be welded or bonded to each other, for example. Alternatively or additionally, the frame halves can also comprise, for example, projections and grooves or similar areas permitting that the two frame halves interlock.

FIG. 6 illustrates a cross-section through the hollow frame profile of the frame of FIG. 1 in the area of the head portion 1. As can be clearly seen in FIG. 6, the wall thickness of the hollow frame profile can quite considerably vary according to the invention along a cross-section through the hollow frame profile. In the depicted preferred embodiment, the frame profile comprises an outer portion 7, an inner portion 6 and two side portions 8 a and 8 b in the area of the head portion, wherein in the depicted cross-section the wall thickness of the inner portion 6 and the outer portion 7 is considerably greater than the wall thickness of the two side portions 8 a and 8 b. There is a continuous and/or gradual transition of the wall thickness from the inner and/or outer portion to the two side portions. Such a finely adjusted cross-sectional profile could not be achieved by means of the conventional tube blowing process. However, when magnesium and/or magnesium alloys are injection molded, such a cross-sectional profile can be configured in a simple way by means of the respective configuration of the injection mold, wherein the cross-sectional profile can additionally vary along the frame contour in the longitudinal direction.

FIG. 7 illustrates a cross-section through the hollow profile in the area of the bridge 5. The wall thickness of the hollow profile varies in this area, too, even though the variation is not as distinct as in the example of FIG. 6.

An additional variation in the wall thickness in the longitudinal direction of the frame, i.e., along the frame contour, is illustrated in FIG. 8, which shows a longitudinal section through the handle portion 2 of the frame of FIG. 1. As can be clearly seen here, the wall thickness of the hollow frame profile significantly increases in the area of the transition to the branch 4 b.

According to the invention, it is not the entire frame that has to comprise magnesium. It is rather possible according to the invention that the frame comprises a portion or a separate component that is made of magnesium or a magnesium alloy. A preferred embodiment of such a separate component from a magnesium alloy is depicted in FIGS. 9 to 13. This separate component is a section from the throat area, namely the bridge 5 as well as portions of the branches 4 a and 4 b and in the transition from the throat portion into the head portion. During the game, the stress of this frame part is particularly complex so that the greatest effect by means of a variation in the wall thickness of the hollow profile can be achieved in this area. According to the invention, it is, however, also possible to produce other insert components from magnesium or a magnesium alloy, such as, for example the handle portion or areas of the head portion, and incorporate them into the frame according to the invention. To this end, the separate magnesium component preferably is pressed together with the prepreg layers of the remaining frame by mans of the tube blowing process. As can be seen, for example, in the side view of FIG. 10, the injection molding operation can be controlled by means of the process according to the invention so well that even the string holes 10, through which the strings forming the stringbed are passed, can be accordingly produced without any additional drilling step being required. 

1. A frame for a ball-game racket comprising a head portion and a handle portion, wherein at least a part of the frame is configured as a hollow profile and comprises magnesium and wherein the wall thickness of the hollow profile varies along a cross-section through the hollow frame profile.
 2. The frame according to claim 1, wherein further the wall thickness of the hollow profile varies in the longitudinal direction of the frame.
 3. The frame according to claim 1, wherein the variation in the wall thickness along at least one cross-section and/or longitudinal section relative to the minimum wall thickness is at least 25%, preferably at least 50%, more preferably at least 75%, particularly preferably at least 100%.
 4. The frame according to claim 1, wherein the wall thickness of the frame in sections is thinner than 1 mm, preferably thinner than 0.8 mm, more preferably thinner than 0.7 mm, particularly preferably thinner than 0.6 mm.
 5. The frame according to claim 1, wherein the frame profile of the frame comprises an outer portion, an inner portion and two side portions in the head portion, and wherein the wall thickness of the inner and/or outer portion at least in sections is thicker than the wall thickness of the side portions.
 6. The frame according to claim 1, wherein the frame profile of the frame comprises an outer portion, an inner portion and two side portions in the head portion, and wherein the wall thickness of the inner and/or outer portion at least in sections is thinner than the wall thickness of the side portions.
 7. The frame according to claim 1, wherein the frame profile of the frame comprises an outer portion, an inner portion and two side portions in the head portion, and wherein the wall thickness of the inner portion at least in sections is thinner than the wall thickness of the outer portion.
 8. The frame according to claim 1, wherein the frame comprises at least 50% (by weight), preferably at least 80% (by weight), more preferably at least 90% (by weight), particularly preferably at least 95% (by weight) of magnesium.
 9. The frame according to claim 1, wherein the frame comprises a magnesium alloy, preferably a fiber-reinforced or particle-reinforced alloy, particularly preferably SAE SiC/AZ91.
 10. The frame according to claim 1, wherein the frame is locally reinforced with carbon fiber material and/or glass fiber material.
 11. The frame according to claim 10, wherein the frame is reinforced with carbon fiber material and/or glass fiber material in the area of the transition between the head portion and the handle portion.
 12. The frame according to claim 1, wherein the frame comprises two frame halves which are welded to each other, pressed together and/or bonded to each other.
 13. The frame according to claim 12, wherein the two frame halves are identical and wherein each of the two frame halves is asymmetrical with respect to the longitudinal axis of the frame.
 14. The frame according to claim 1, wherein the frame comprises a frame portion comprising magnesium, and wherein the wall thickness of the hollow profile varies along a cross-section through this frame portion.
 15. The frame according to claim 14, wherein the frame in said frame portion comprises at least 50% (by weight), more preferably at least 80% (by weight), more preferably at least 90% (by weight) and particularly preferably at least 95% (by weight) of magnesium.
 16. The frame according to claim 14, wherein said frame portion comprises two frame portion halves which are welded to each other, pressed together and/or bonded to each other.
 17. A process for producing a frame for a ball-game racket, in particular a frame according to claim 1, wherein the process comprises the following steps: providing an injection mold and a core, wherein the core consists of a material having a melting temperature below 400° C.; casting the frame by inserting a material comprising magnesium between the injection mold and the core, preferably by means of the thixo process; and melting the core by heating the racket including the core to above the melting temperature of the core, wherein the heat resistance temperature of the frame is not exceeded.
 18. The process according to claim 17, wherein the frame material comprises at least 50% (by weight), preferably at least 80% (by weight), more preferably at least 90% (by weight), particularly preferably at least 95% (by weight) of magnesium.
 19. The process according to claim 17 or 18, wherein the frame material comprises a magnesium alloy, preferably a fiber-reinforced or particle-reinforced alloy, particularly preferably SAE SiC/AZ91.
 20. The process according to claim 17, wherein the core material comprises or consists of one of or a combination of the following materials: an alloy on the basis of tin, an alloy on the basis of bismuth, an alloy on the basis of lead, an alloy on the basis of zinc, an alloy on the basis of indium, glass, plastics with or without glass-fiber reinforcement, polyamide. 